(1) Field of the Invention
The present invention relates to a novel welded can and a process for the preparation thereof. More particularly, the present invention relates to a novel welded can formed of a surface-treated steel plate having a layer of metallic tin or a tin-iron alloy and a hydrated chromium oxide layer formed thereon, in which the corrosion resistance, lacquer adhesion and appearance characteristics are highly improved by the presence of these covered layers on the inner and outer surfaces of the weld seam, and a process for the preparation of this novel welded can.
(2) Description of the Prior Art
As the conventional can-forming method ordinarily adopted in the art, there can be mentioned a method in which a can blank such as tinplate is formed into a cylinder and both the side edges are bonded together by lap seaming, lock seaming or combination thereof using a solder or adhesive. This conventional method, however, is defective in that a considerable area is necessary for the seam portion, and the method is not preferred from the viewpoint of saving of resources. Moreover, the bonded can prepared according to this method is insufficient in the strength and durability of the seam. Furthermore, when the seam is formed by using a solder or adhesive, since there is present a considerable step difference in the side seam, leakage is readily caused in this step portion if a can end is double-seamed to the can body.
As the can-forming method that has heretofore been adopted instead of the method using a solder, there can be mentioned a draw-ironing method. Although so-called seamless cans prepared according to this method are used in some fields, these seamless cans cannot be used at all as vacuum cans, that is, cans which are subjected to retort sterilization after packing of contents, because the side wall of the can body is drastically deformed under pressure in these seamless cans. As another instance of the can-forming method that can be used instead of the method using a solder, there is known a method in which both the side edges of the can blank are lap-bonded together by welding. In a so-called welded can prepared according to this method, the area of the lap seam portion is smaller than in the soldered can and the thickness of the seam portion is relatively thin. Therefore, the above-mentioned problem of the step difference is moderated in the welded can, and furthermore, the welded can is advantageous over the bonded can, because a particular bonding agent such as a solder or adhesive need not be used. However, when a certain kind of a can stock is used, the can-forming operations become complicated, and the conventional welded cans are still insufficient in the corrosion resistance, lacquer adhesion and appearance characteristics of the seam.
A welded seam can is ordinarily prepared by forming a can stock into a cylinder, and subjecting the side edges to be lapped of the cylinder to electric resistance welding by passing the side edges between a pair of upper and lower electrode rollers optionally through an electrode wire.
Tin-free steel obtained by subjecting a cold rolled steel plate to an electrolytic treatment with chromic acid, which is used as a can stock, is cheaper and more readily available than other can stocks such as tinplate, and tin-free steel is advantageous in that it is excellent in the corrosion resistance and lacquer adhesion. However, this tin-free steel (hereinafter referred to as "TFS") involves the following problem. Namely, since a chromium oxide layer having a high electric resistance is inevitably present on the outer surface, flow of an electric current is inhibited by this layer when the above-mentioned electric resistance welding is carried out. Therefore, a troublesome operation of removing this chromium oxide layer covered on the steel substrate should be conducted prior to the welding operation, with the result that the conventional method of preparing welded TFS cans has a disadvantage in that the number of steps in the process is increased. Furthermore, since the steel substrate is exposed in the bonded area by removal of the chromium oxide layer, the corrosion resistance and lacquer adhesion are inevitably degraded in this area. Removal of the chromium oxide layer is ordinarily accomplished by mechanically cleaning the portions to be lapped of the can stock. At this cleaning step, the chromium oxide layer or coated lacquer in other areas is damaged by pieces or particles separated from the can stock by cleaning, and another problem of incorporation of these pieces or particles into the packed content arises.
When a conventional tin-plated steel plate, that is, tinplate, is used as the can stock, the above-mentioned problem of inhibiting of flow of a welding current does not arise, but tin is a very expensive metal, exhaustion of which is a world-wide problem, and it is eagerly desired to reduce the amount of tin. However, reduction of the amount of tin should naturally result in reduction of the corrosion resistance in the obtained cans, and especially in case of welded cans, extreme increase of a contact resistance between the can stock and the electrode roller or electrode wire is brought about by reduction of the amount of tin and various disadvantages are developed. For example, the temperature of the inner and outer surfaces of the lapped portion is extremely elevated to cause splashing (a phenomenon of extrusion of molten iron from the seam), the protecting effect is degraded or lost by scattering of fumes from the plated tin layer and oxidation is promoted by such high temperature, with the result that the appearance characteristics, corrosion resistance and lacquer adhesion of the seam are degraded.